The official death toll from the suicide bomb blast that ripped through a luxury hotel in Islamabad yesterday has risen to 53, the Pakistani prime minister, Yousuf Gilani, said today.

The blast, one of the biggest attacks in Pakistan for over a decade, happened at the Marriott hotel at around 8pm. The hotel was left burning fiercely all along its façade, while other buildings in the vicinity were also left damaged.

Witnesses said security staff at the front of the hotel, where the blast was strongest, had 'simply been vaporised'.

Scores of bodies were being brought out of the flaming building as rescue workers battled the blaze in scenes of chaos.

Victims included the Czech ambassador, it emerged today. Two Americans and a Vietnamese national were also killed. Eleven other foreigners were among the 266 wounded in the blast, Rehman Malik, the top official in the interior ministry, told Reuters.

At least four Britons were injured in the attack, two of them children. Officials told Reuters that a British woman remained in hospital.

Rescue teams were today searching the blackened hotel but temperatures remained high and fires were still being put out in some parts. Officials said the main building could still collapse.

In a televised address to Pakistan today, the president, Asif Ali Zardari, said: "This is an epidemic, a cancer in Pakistan which we will root out. We will not be afraid of these cowards."

The foreign secretary, David Miliband, condemned the strike as "yet another shocking and disgraceful attack without justification". He said such a "brutal act of terror deserves the condemnation of the entire international community" and added that the British government would continue "to stand shoulder to shoulder with the government of Pakistan against the violent extremists who have no answers, but only offer death and mayhem".

The US president, George Bush, said the attack was "a reminder of the ongoing threat faced by Pakistan, the United States, and all those who stand against violent extremism".

Hotel staff said that all the Marriott's function rooms, including the large ballroom, had been hired for iftar - the traditional communal meal that breaks the day-long fast that Muslims observe during the holy month of Ramadan. According to the hotel owner, at least 700 people would have been in the hotel at the time of the blast at 8pm. Around 300 eating under a marquee at the back of the hotel away from the blast survived.

The senior police official Asghar Raza Gardezi said the explosion was caused by more than a tonne of explosives, probably delivered in a small truck. Other reports indicated a series of bombs - possibly one small explosion that paved the way for other, larger, blasts.

Imtiaz Gul, a journalist, was dining with friends in the marquee when the attack happened. "First there was a blast and then all the electricity went out ... then it came back on and there was a second much bigger blast or a series of deafening blasts," he said.

"We found our way out but it was really harrowing. It was carnage, there was debris everywhere, body parts, glass. At the front of the hotel there were not even body parts. The blast had just destroyed everything."

Other witnesses spoke of corpses strewn on the ground. Scores of ambulances rushed to the scene, negotiating burnt-out vehicles and a vast crater left by the explosion. Windows in buildings nearly a mile away in residential areas and a heavily guarded compound where ministers have their official homes were damaged.

The Marriott is in the centre of the city, close to the National Assembly, the main commercial thoroughfare and the national television headquarters. Security has been high at the hotel since a previous attempted suicide bombing in 2007, foiled by a security guard.

A US state department official using a section of white pipe as a walking stick was seen leading three colleagues through the rubble from the charred building, one of them bleeding heavily from a wound on the side of his head.

One of the four, who identified himself only as Tony, said they had begun moving towards the rear of the hotel's Chinese restaurant after the first blast when the second threw them against the back wall. "Then we saw a big truck coming through the gates," he said. "After that it was just smoke and darkness."

There was no immediate claim of responsibility for the blast, though radical militants allied with the Taliban and al-Qaida who are engaged in a violent insurgency in Pakistan will be the prime suspects.

The use of multiple truck bombs is a favoured technique of Islamic militants. A security guard at the scene said he saw a large vehicle that caught fire on its front before suddenly exploding.

Pakistan, a key US ally in the war on terror, has faced a wave of militant violence in recent weeks following offensives by its army against insurgents in the restive regions along its frontier with Afghanistan. Though the capital has avoided most of the bloodshed, hundreds have died in a series of bombings and suicide attacks in the last six months. Benazir Bhutto, the former prime minister, was killed in December in a bomb and shooting attack and there have been strikes in most major cities. Earlier this month the convoy of the prime minister was shot at.

Hours before yesterday's blast, Bhutto's widower, Zardari, gave a speech to parliament. He pledged Pakistan's support in the international fight against terrorism, continuing the policy of his predecessor in the post, General Pervez Musharraf.